This invention relates to gamma ray scattering or gamma-gamma density well logging techniques and, more particularly, to such techniques for simultaneously deriving a compensated formation bulk density and a measurement of the thickness of casing in a cased well borehole.
Well workovers in relatively old wells in which casing has been set have become fairly routine in the maintenance of existing oil fields. In performing well workovers it is necessary when relogging the well to utilize nuclear measurement techniques which can measure formation characteristics through the casing in a cased well borehole. One such technique, which is being increasingly used, has been the gamma ray scattering or gamma-gamma density measurement of the formation bulk density as measured by the scattering characteristics for relatively low energy gamma radiation.
In gamma-gamma logging, a gamma ray source is utilized to irradiate formations through the casing. The scattered gamma rays impinging upon a pair of spaced gamma ray detectors produce count rate signals which may be interpreted in terms of the electron density or formation bulk density of the earth formations behind the casing. This measure of bulk density is used to determine many formation parameters of interest. As an example, bulk density along with the response of a neutron log can be used to identify gas saturated formations.
In working over or recompleting older existing wells, in which casing has been set, it is also very desirable to the well operator to be able to determine the condition of the casing. The gamma ray scattering measurement when processed according to the techniques of the present invention is capable of yielding information concerning the casing thickness simultaneously with the measurement of the formation bulk density. This casing thickness information may be utilized, for example, to locate casing collars in the wellbore or to indicate thin casing sections which could be a problem in the recompleted well after it goes onto production. By anticipating casing weakness ahead of time, corrective measures such as squeeze cementing may be taken to reinforce weakened casing sections.